The present invention relates to optical devices for use in coupling light from one optical fiber to another, and to the use of such devices for optical communications.
Low insertion loss, wavelength-selective optical couplers are important components for optical fiber communications, especially for optical communications systems that use wavelength-division multiplexing (WDM). WDM systems transmit many optical channels in one fiber, with each channel being distinguished by its central wavelength. For efficient operation of a WDM communication system, the system should include the abilities to selectively add, drop and reroute channels. Ideal wavelength-selective couplers should have low insertion loss ( less than 1 dB), high channel isolation ( greater than 30 dB), low back-reflection, and low cost. A number of wavelength-selective couplers are currently commercially available, but it is believed that none of the currently used couplers adequately meet all of the above desired characteristics.
One configuration for selecting a single channel from many channels in a fiber uses a 2xc3x972 coupler with a fiber Bragg grating affixed to one of the coupler""s outputs (e.g., Kashyap et al., Electronics Letters 26, p. 730 (1990)). Multi-wavelength input light enters one the coupler""s inputs and becomes split between the two outputs. The Bragg grating at one output is chosen to reflect the desired wavelength, and light at this wavelength is emitted from the second input. This device exhibits high loss (3 dB) for all of the unselected channels and even higher loss (6 dB) for the selected channel. Such loss will typically by unacceptably high for commercial applications.
Another approach uses an optical circulator in combination with a Bragg grating at the circulator output, P.C. Becker et al., Erbium Doped Fiber Amplifiers, pp. 55-58, Academic Press (1999)). While such a configuration has a smaller insertion loss (about 2 dB), the high cost of circulators makes this device expensive.
Bilodeau et al. (Photonics Technology Letters 7, p. 388 (1995)) fabricated a fiber Mach-Zehnder interferometer that served as a wavelength-selective coupler. In order to add or drop a channel, this device relies on a precisely adjusted phase difference between two interferometer arms. This design makes the device undesirably sensitive to environmental conditions, especially temperature.
Other coupler designs use evanescent coupling of light between two fibers within a tapered region of a fused coupler. Snitzer (U.S. Pat. No. 5,457,758) used Bragg gratings to redirect the selected wavelength of light into a separate output of the coupler. Kewitsch et al. (U.S. Pat. No. 5,805,751) used a coupler with two dissimilar fibers. A Bragg grating inscribed within one of the fibers coupled light into a backward-propagating mode of the second fiber; non-resonant wavelengths were not affected and propagated with small losses through the first fiber. However, making a wavelength-selective coupler based on a fused fiber coupler requires very uniform fusion of two fibers over the length of the coupling region, making such devices difficult to manufacture.
Unlike a short-period Bragg grating, a long-period grating can couple light from a core mode into a different forward-propagating core mode (Hill et al., U.S. Pat. No. 5,216,739) or into a forward-propagating mode of the cladding (Vengsarkar, U.S. Pat. No. 5,430,817). Vengsarkar and Walker (U.S. Pat. No. 5,550,940) proposed a fused coupler-based device that uses a long-period grating. In that device, the cores of two optical fibers are spaced sufficiently far apart so that, in the absence of any gratings, there is negligible coupling of light between the two fiber cores. A single long-period grating inscribed in one fiber""s core couples input light into a common cladding. A fraction of light in the cladding mode will then couple into the second fiber core provided that the interaction length is very small. This restriction limits the amount of light coupling and broadens the width of the coupling resonance.
A wavelength-selective optical coupler according to the present invention uses two or more gratings in two or more waveguides, such as optical fibers, to transfer light at a desired and selected wavelength from the core of one optical fiber into the core of another optical fiber through a coupling region that may include a common cladding or two claddings positioned close together to define a coupling region between them.
The wavelength-selective coupler of the present invention may use short-period (Bragg) gratings, but preferably uses long-period gratings. Alternatively, some of the gratings could be short period gratings and some of the gratings could be long period gratings. In an optical fiber, short-period gratings couple light from one mode to a counterpropagating mode. The period of a short-period grating in an optical fiber ranges from approximately 0.1 micron to 2 microns. In contrast, long-period gratings couple light between modes traveling in the same direction down a length of optical fiber. The period of long-period gratings falls in the range of approximately 10 microns to 1000 microns. Because long-period gratings couple light into forward-propagating modes, there is little back-reflected light. Long-period gratings do not cause loss for non-resonant wavelengths, so the insertion loss of the invented coupler is limited only by its splicing loss, which can be reduced to less than 0.1 dB. Channel isolation can be very high ( greater than 40 dB) because the device uses three stages of coupling (core-cladding, cladding-cladding, cladding-core) which effectively filter out non-resonant light. Temperature dependence of the resonance wavelength of the long-period gratings can be eliminated by proper design of the fiber, as disclosed, for example, in U.S. Pat. No. 5,703,978.
A number of different configurations can be employed. In each case, there are two fibers, each with a core, and with either a separate cladding for each of them or a common cladding. The fibers are positioned close together over a common length in a region referred to as the coupling region. The fibers may be wrapped and held together, but preferably are not fused together because such a fusing process can make it difficult to fabricate uniform gratings in the fibers. The coupling region preferably keeps the claddings no more than 10 microns apart over a distance of about 1 mm-500 mm in a lengthwise direction. The gratings can be formed in the cores or in the claddings of the respective fibers, and can be designed to obtain the desired spectral dependence of coupling. The grating can be apodized to reduce coupling at unwanted wavelengths.
Variations and additions are possible, including the addition of a third fiber with a core, cladding, and grating, for providing light at a desired wavelength from the third fiber to a first of the fibers, while the second fiber has a coupling region with the first to receive light at another desired wavelength. With just two fibers having a coupling region, gratings in the first fiber on either side of the coupling region and a grating in the second fiber at the coupling region, a total of two fibers can be used to both add and drop light at a desired wavelength or at different wavelengths.
These couplers can be made tunable in wavelength by varying the periodicity of the long-period gratings or the refractive indices of the cores or claddings. Similarly, the devices can vary the intensity of the light in the fibers by varying the strength of one or more of the gratings. Alternatively, the cladding modes can be altered to vary the coupling between them. For example, the attenuation of the cladding modes or the propagation constant of the cladding modes can be varied. The coupling between modes can be varied by altering the strength of one or more of the gratings.
The present invention relates not only to the use of circular optical fibers, but also to any other geometries of fibers or other waveguides that incorporate a waveguiding cladding.
Preferably the fibers should be single-mode at the wavelength of operation. Alternatively, the fiber cores could support more than one mode. Any number of fibers may be placed in optical contact with each other, or in close proximity, so that there is coupling between their cladding modes. There can by an arbitrary number of input fibers, as well as an arbitrary number of output fibers. Multiple long-period gratings may be inscribed within each fiber for coupling multiple channels.
The present invention thus provides a wavelength-selective optical coupler that presents negligible loss to all unselected channels, that has negligible sensitivity to temperature variations, that can be manufactured economically, that has high channel isolation, and that has no back-reflection. The wavelength selective coupler of the present invention can be used to make an add/drop filter to add light at a desired wavelength, filter out light at a desired wavelength, or both, especially for WDM communications systems. The present invention has a wide range of applications, including combining multiple light sources into one fiber. In particular, several pumping lasers having different wavelengths may be combined in a single fiber thereby increasing the intensity of light in that fiber for efficient pumping of a fiber laser. The present invention can also be used, for example, to multiplex signals in the 1550 nm and 1300 nm communications bands.
According to the present invention, a grating can also be used to couple light from an optical fiber to a planar waveguide and back without fiber termination. A grating in the fiber couples light from the core to the cladding. The waveguide is put next to the fiber, and the light is coupled from the cladding to the waveguide.
Other features and advantages will become apparent from the following detailed description, drawings, and claims.